Subject: [manet-dlep-rg] DLEP and multihop layer 2 mesh networks

From: "Dowdell, John" <[email-masked]>
Date: Mon, 2 Sep 2013 14:46:00 +0000

In response to the action to investigate layer 2 mesh network radios with regard to DLEP, I have the following to offer based on experience within the design team that I work with. For ease of comparison I have used Teco's format.

1. Scope.

The information below applies to mesh networking radios we have worked with. Other mesh radios are available, but as there appears to be no common standard across radios on the market, differences may exist between these and other radios. The ones we have used are omni-directional medium to high capacity intended for mounted or dismounted use on the ground or in the air. Directional capability may be enabled through different antenna systems. A radio becomes a member of the mesh by means of some pre-agreed configuration. A radio may send traffic from an attached router to others, may receive traffic and pass to an attached router, and may forward traffic to other radios if it has received traffic that is not for its attached router.

2. Topology.

The radios we have used are broadly similar to 802.11s stations. Other radio technologies that may be of use include 802.15.x, 802.16 and 802.20, as well as any proprietary technologies. The radios accept IP packets and the mesh appears to other networking equipment as a single broadcast domain, regardless of how many radios are traversed between source and destination. The radios are collectively responsible for all forwarding operations and for working out an appropriate path over which to forward traffic. From outside the radio domain, there is the temptation to try to control the direction of the appropriate path, but experimentation suggests that this is a function best left to the radio manufacturer.

3. Interface to External Router.

The radio is typically connected to the router using fast Ethernet. Since the mesh radio appears as a single broadcast domain, the mesh radios do not have to support a routing protocol. The radios will typically have an IP address so that they can be managed, but traffic that is to be forwarded on to the RF media is simply forwarded to the radio function. IPv6 may well be supported even if not advertised because frames are simply passed through.

4. DLEP Metrics

The radios typically form a mesh based using an agreed configuration of channels, security codes, etc., and all nodes are notionally identical. All radios can forward layer 2 frames, with or without an attached router. A radio without an attached router simply performs a forwarding function for other radios, apart from management traffic. If all of the radios in a particular group are within range of each other, then this is identical to the situation expressed by Rick in his posting [1], where all radios can directly forward frames to their peers. When one or more radios in the group move out of range of some, but not all, of the other radios, then one or more radios have to take responsibility for acting as layer 2 forwarding agents.

Two DLEP metrics can usefully be calculated, latency and bandwidth. Latency is simply the delay between source and destination radio, which may include a forwarding radio. Bandwidth is a much more difficult entity to measure precisely. What is clear is that in mesh networks where a sender and receiver are out of RF range of each other, and one or more forwarding nodes are employed, the RF environments  between sender and forwarder, and between forwarder and receiver could be different.

For example, consider a collection of three radios, designated A, B and C. A and B are within range of each other, as are B and C. However A and C are not within range of each other, and traffic from A to C must pass through B. If the distance between A and B is short relative to the distance between B and C, the RF path loss between B and C will be higher than A and B, and hence the available bandwidth between B and C could be less than that available between A and B, assuming an adaptive modulation scheme is being used.

To be of use to a DLEP-equipped router, the radios must be able to compute (a) the maximum bandwidth available to user traffic on each link A-B and B-C, and (b) if at all possible the remaining bandwidth available after current user traffic has been taken account of. It is suggested that a DLEP agent on radio A should report the maximum bandwidth to B as computed, but the maximum bandwidth to C is the lower of the bandwidths between A-B and B-C. The consequences of over-estimating the bandwidths are that the router allows more traffic on to the mesh than it can withstand, and the radios may become overloaded and take action to preserve their own sanity.

The metric calculation should take into account multicast and multi-QoS traffic if the radios are equipped for those features.

In reality, where there are several radios in a mesh when the mesh starts to separate, the business of calculating "best path" and maximum bandwidths will get complicated. It may be that there are multiple paths to a destination and the path selection algorithm may be able to choose two or more paths along which to forward traffic, thereby increasing possible maximum bandwidth, but to DLEP the neighbour is still a single entry in the table.

- John

[1] http://www.ietf.org/mail-archive/web/manet-dlep-rg/current/msg00012.html
The information contained within this e-mail and any files attached to this e-mail is private and in addition may include commercially sensitive information. The contents of this e-mail are for the intended recipient only and therefore if you wish to disclose the information contained within this e-mail or attached files, please contact the sender prior to any such disclosure. If you are not the intended recipient, any disclosure, copying or distribution is prohibited. Please also contact the sender and inform them of the error and delete the e-mail, including any attached files from your system. Cassidian Limited, Registered Office : Quadrant House, Celtic Springs, Coedkernew, Newport, NP10 8FZ Company No: 04191036 http://www.cassidian.com